Method for the common transmission of digital and analogue modulated radio broadcasting and/or television broadcasting signals

ABSTRACT

A method is described that is used for common transmission of digital- and analog-modulated radio and/or television signals, in particular over a broad band cable system ( 30 ). The method includes transmitting at least one digital radio and/or television signal along with an analog television signal in one and the same channel, in which the frequency spectrum of the at least one digital radio and/or television signal is limited to a frequency range that is less than the width of the at least one channel. To lessen the influence of the at least one digital radio and/or television signal on the analog television signal, the at least one digital radio and/or television signal must fall below a predetermined signal level, and the peak level of the analog television signal must exceed a predetermined value, which is markedly higher than the predetermined signal level of the at least one digital radio and/or television signal. To prevent cross-modulation between analog- and digital-modulated radio and/or television signals, the amplitude of the frequency spectrum of the at least one digital radio and/or television signal must fall below a predetermined value, which is markedly lower than the amplitude of the video carrier of the analog television signal.

BACKGROUND OF THE INVENTION

The present invention relates to a method for common transmission ofdigital-modulated and analog-modulated radio and/or television signals,especially over a broadband cable system.

The journal “Funkschau” [Broadcasting Review] No. 9, 1995, p. 46,discloses a transmission method in which a flexible multiplexer at theoutput of a video server combines a plurality of program signals withdifferent data rates into a transmission package; the digital summationsignal is QAM-modulated (QAM=quadrature amplitude modulation) and isthen transmitted parallel to the analog TV signals already fed into acable. The digital signal then occupies one entire TV channel of thekind that was previously needed for a single analog television program.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodfor common transmission of digital-modulated and analog-modulated radioand/or television signals, especially over a broadband cable system,which provide increased transmission capacity without significantlyincreasing side channel interference.

According to the invention the method for common transmission ofdigital-modulate and analog-modulated radio and/or television signalscomprises the steps of:

a) transmitting at least one digital signal together with an analogtelevision signal in a common channel of at least one communicationschannel;

b) controlling a signal level of the at least one digital radio and/ortelevision signal so that it does not exceed a predetermined digitalsignal level;

c) controlling a signal level of the analog television signal so that itdoes not drop below a predetermined analog signal level which is muchgreater than the predetermined digital signal level of the at least onedigital radio and/or television signal;

d) controlling the at least one digital radio and/or television signalso that an amplitude of a frequency spectrum of the at least one digitalradio and/or television signal does not exceed a predetermined amplitudevalue, which is much lower than an amplitude of a video carrier of theanalog television signal; and

e) controlling the at least one digital radio and/or television signalso that the frequency spectrum of the at least one digital radio and/ortelevision signal is limited to a frequency range smaller than a widthof the common channel.

The method of the invention has the advantage over the prior art thatthe transmission of digital-modulated radio and/or television signalswith an analog television signal in one and the same channel leads to amarked increase in transmission capacity, so that substantially moreradio and/or television programs can be transmitted.

In a preferred embodiment of the method the data quantity of the atleast one digital radio and/or television signal is reduced by coding sothat additional digital radio and/or television signals can betransmitted together with the analog television signal in the onechannel, thus further increasing the transmission capacity.

It is also advantageous to utilize the channel for transmitting furthersupplementary data. Once again, still a further increase in transmissioncapacity is thereby achieved.

By suitably disposing the frequency range of the at least one digitalradio and/or television signal between two carrier frequencies, possibleinfluences of the digital and analog signals on one another canadvantageously be kept slight and below a threshold of perceptibility.

By transmitting audio carrier frequency components of the analogtelevision signal with predetermined attenuation it is possible in apreferred embodiment to obtain an improvement in the signal to noiseratios for the digital signals by attenuation of interfering video andaudio carrier frequency components of the transmitted analog radioand/or television signals.

In a preferred embodiment of the method the individual digital radioand/or televisions signals are combined into blocks in the at least onechannel and the frequency ranges of at least two of the transmitteddigital radio and/or television signals are separated from one anotherby a protective frequency spacing. The advantage of this feature is thatmutual interference between the signals is avoided.

An advantage in the modulation of one or more digital radio and/ortelevision signals combined into a multiplex signal in accordance with apreferred embodiment of the method is the selection of carrierfrequencies that for at least one audio carrier frequency, colorauxiliary carrier frequency or video carrier frequency of the analogtelevision signal do not fall below a predetermined frequency spacing.In this way, because critical video and audio carrier frequencies areblanked out, interference of carrier frequencies on the analogtelevision signal with the spectrum of the digital multiplex signal isprevented.

The frequency range of one or more digital radio and/or televisionsignals, combined into a multiplex signal, is advantageously splitbetween video and audio carrier frequencies of the analog televisionsignal, and the corresponding video and audio carrier frequencies of theanalog television signal are advantageously separated by one protectivefrequency spacing each. In this way, even digital multiplex signals thatbecause of the width of their frequency range can be accommodated in thechannel of the analog television signal only with difficulty withoutinterfering superposition with video and audio carrier frequencies ofthe analog television signal, can be accommodated entirely withoutproblems between the individual interfering video and audio carrierfrequencies of the analog television signal because of the describedsplitting of their frequency range, for instance by blanking out carrierfrequencies of the multiplex signal, thus averting their superpositionon one another, which would cause interference.

In a preferred embodiment advantageously the transmission of one or moredigital radio and/or television signals, can be combined into amultiplex signal, in an arbitrary direction. In this way, via the broadband cable system, interactive radio or television and/ortelecommunications can be carried out, above all including theutilization of the transmission of digital supplementary data inaccordance with the preferred embodiment described above.

Another advantage of this preferred embodiment is also the realizationof reverse data channels in broad band cable systems, without requiringadditional frequencies or without having to change the conventionalfrequency allocation of the broad band cable system.

An apparatus transmitting and receiving digital multiplex signalsmodulated according to an OFDM method, preferably digital radio and/ortelevision signals, comprises, for reception, a tuner circuit forselecting a channel of the transmitted frequency band, a demodulator fordemodulating the OFDM modulated signals, a decoder for error evaluationof the received digital signals, a demultiplexer for splitting thecorresponding multiplex signal into individual digital signals and aninterface device for connection of data playback devices and/or opticalbroadband distribution network, especially using fiber optic devices;and, for transmission, a channel adaptation unit for coding andcombining individual digital signals transmitted via the interfacedevice from a data playback device and/or from the optical broadbanddistribution network connected to the interface device into a multiplexsignal; a modulator for modulating the multiplex signal at carrierfrequencies in frequency channels, especially for transmission of analogtelevision signals and a coupler for connection to a broadband cablesystem, which is connected to the tuner circuit and the modulator. Theapparatus according to the invention has the advantage that interactiveradio or television and/or telecommunications using digital multiplexsignals modulated by an OFDM method is enabled by coupling bothreceiving and transmitting components to the broad band cable system.

Advantageous preferred embodiments of this apparatus are possible. Anespecially advantageous feature is the reduction of the data rate of thedigital multiplex signal, in that signal components of the multiplexsignal to be transmitted are to be modulated to the carrier frequencieswhich exceed a predetermined frequency spacing from at least one audiocarrier frequency and/or one color auxiliary carrier frequency and/orone video carrier frequency of an analog television signal. In this way,with the apparatus, multiplex signals that are not interfered with bythe video and audio carrier frequencies of an analog television signalcan be generated.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

FIG. 1 is a block diagram of an apparatus for feeding digital and analogradio and/or television signals into a broadband cable system;

FIGS. 2 to 4 are graphical illustrations of respective examples ofsignal spectra in an analog channel;

FIG. 5 is a block diagram of an apparatus for receiving transmitted datafrom the broadband cable system;

FIG. 6 is a block diagram of a transmitting and receiving apparatusaccording to the invention for OFDM-modulated digital multiplex signals;

FIG. 7 is a graphical illustration showing a superposition of thespectrum of an analog television signal with a frequency range, splitinto three frequency blocks, of an OFDM-modulated digital multiplexsignal; and

FIG. 8 is a graphical illustration showing protective frequency spacingsof FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, reference numeral 5 designates a multiplexer, to which via afirst encoder 1 a digital television signal and via a second encoder 2 adigital radio signal are delivered. Via an enciphering unit 10, amodulator 12 and a first amplifier 13, the multiplexer 5 is connected toa mixer 15, to which an analog television signal is delivered via asecond amplifier 14. The signal generated in the mixer 15 is fed into abroad band cable system 30, of which only one cable input is shown inFIG. 1, via a third amplifier 20 and a filter 25 for audio carrierfrequency components of the analog television signal. The output of thefirst amplifier 14 represents the output of a first transmitter fortransmitting digital-modulated radio and/or television signals. Theoutput of the second amplifier 14 then represents the output of a secondtransmitter for transmitting analog or analog-modulated radio and/ortelevision signals. The digital- and analog-modulated radio and/ortelevision signals may also be transmitted from a single transmitter,which then includes both the output of the first amplifier 13 and theoutput of the second amplifier 14, and whose output is the output of thefilter 25.

The analog television signal delivered to the second amplifier 14 isamplified in the amplifier 14 to a signal level, so that the peak levelof the analog television signal exceeds a predetermined value, forinstance of 65 dB μV. The digital signals delivered to the multiplexer 5via the first encoder 1 and the second encoder 2 are combined,frequency-multiplexed, into a digital signal in the multiplexer 5 andthen enciphered in the enciphering unit 10. The two encoders 1 and 2reduce the quantity of data in the digital television signal and thedigital radio signal, so that a limitation of the frequency spectra ofthe digital signals is accomplished, each to a predetermined frequencyrange that is less than the width of the channel of the analogtelevision signal. To reduce the data quantity in the digital televisionsignal, data compression algorithms are suitable, such as MPEG1, MPEG2or MPEG4 (MPEG=motion picture expert group). For audio data reduction,the MPEG standard is equally suitable. The enciphered digital signal isthen, in the modulator 12, subjected to a modulation process, such asthe COFDM process (COFDM=coded orthogonal frequency divisionmultiplexing), the PSK process (PSK=phase shift keying), or the QAMprocess. The goal of this provision is to limit the amplitude of thefrequency spectrum of the digital signal to a predetermined value thatis markedly less than the amplitude of the video carrier of the analogtelevision signal, so that if at all possible cross-modulation with theanalog television signal does not occur. Next, in the first amplifier13, the modulated digital signal is adjusted in level to a value that is20 to 30 dB lower than the value predetermined for the peak level of theanalog television signal. In the mixer 15, the digital signal issuperimposed on the analog television signal. The resultant signal isthen adjusted in its level to the predetermined values via the amplifier20. Finally, the filter 25 is used to lower the signal level in therange of the audio carrier frequencies of the analog television signalby a predetermined value that has no significant influence on theplayback or reproduction quality of the received sound for the user. Inthe exemplary embodiment described, such attenuation can amount to up to10 dB compared with the typical audio levels in broad band cablesystems. This provision improves the reception of the digital signal orits signal to noise ratio. This plays a role above all whenever thefrequency range of the digital signal is superimposed on the frequencyrange of the audio carrier. The task of the modulator 12, finally, isalso to convert the frequency ranges of the digital signals into thechannel of the analog television signal. Depending on the range of thebroad band cable system 30, this broad band cable system has additionalamplifiers and filters for audio carrier frequency components, so thatthe transmitted signal will remain adjusted in its level to thepredetermined values.

It is also possible for a digital signal additionally to be impressed,as described above, on the analog television signal that is broadcastvia a transmission system; the digital signal can be broadcast either bythe same transmission system or some other transmission system.

Along with a digital radio or television program, it is also possible totransmit digital supplementary data. These supplementary data includetraffic reports, transportation schedules, weather reports, or theaterand movie schedules.

FIG. 2 shows one example of a frequency spectrum of the channel of theanalog television signal. The amplitude A is plotted over the frequencyf. The channel is defined by a lower limit frequency f₀₁ and an upperlimit frequency f₀₂. The spectrum 35 of the analog television signal islocated in this channel and has a video carrier at a frequency f₁ and acolor auxiliary carrier at a frequency f_(FHT) which is greater than thevideo carrier frequency f₁. The spectrum 35 also has a first audiocarrier at a frequency f_(T1) and a second audio carrier at a frequencyf_(T2). Both audio carrier frequencies f_(T1) and f_(T2) are greaterthan the color auxiliary carrier frequency f_(FHT). The second audiocarrier frequency f_(T2) is greater than the first audio carrierfrequency f_(T1). The video carrier at the frequency f₁ has the highestamplitude, followed by the amplitudes of the two audio carriers at thefrequencies f_(T1) and f_(T2) and the color auxiliary carrier at thefrequency f_(FHT). Between the video carrier at the frequency f₁ and thecolor auxiliary carrier at the frequency f_(FHT), the spectrum of theanalog television signal has a relatively high amplitude in comparisonwith the range between the color auxiliary carrier at the frequencyf_(FHT) and the upper limit frequency f₀₂, except for the two audiocarriers at the frequencies f_(T1) and f_(T2). Above the color auxiliarycarrier frequency f_(FHT), a spectrum 40 of a digital radio and/ortelevision signal between a lower limit frequency f_(D1) and an upperlimit frequency f_(D2) is superimposed on the spectrum 35 of the analogtelevision signal. In this way, the spectrum 40 of the digital signal isaccommodated in a range of the channel in which, with the exception ofthe two audio carriers at the frequencies f_(T1) and f_(T2), thespectrum of the analog television signal has a relatively smallamplitude, so that interference of the digital signal by the analogtelevision signal is kept as slight as possible. The lower limitfrequency f_(D1) of the spectrum 40 of the digital signal is lower thanthe two audio carrier frequencies f_(T1), and f_(T2), and the upperlimit frequency f_(D2) of the spectrum 40 of the digital signal ishigher than the two audio carrier frequencies f_(T1) and f_(T2). The twoaudio carriers are thus superimposed on the spectrum 40 of the digitalsignal. The lower limit frequency f_(D1) of the spectrum 40 of thedigital signal is higher than the color auxiliary carrier frequencyf_(FHT), so that there is no impairment of the spectrum 40 of thedigital signal by video and color carriers of the analog radio signal.The upper limit frequency f_(D2) of the spectrum 40 of the digitalsignal is lower than the upper limit frequency f₀₂ of the channel, sothat there is no impairment of the spectrum 40 of the digital signal bya video carrier at a frequency f₂, which is higher than the upper limitfrequency f₀₂ of the channel and is thus located in a neighboringchannel. Since the amplitudes of the two audio carriers at thefrequencies f_(T1) and f_(T2) are greater than the amplitude of thespectrum 40 of the digital signal, the two audio carriers do notinterfere with the digital signal. By feeding the audio carriers intothe cable system 30 at predetermined attenuation, this interference isalready lessened. A further reduction in interference takes place in thereceiver.

In a further exemplary embodiment in accordance with FIG. 3, thespectrum 40 of the digital signal is superimposed on the otherwiseunchanged spectrum of the analog television signal, in such a way thatit is located between the second audio carrier frequency f_(T2) and theupper limit frequency f₀₂ of the channel and is thus also no longerimpaired by the two audio carriers.

In a further exemplary embodiment in accordance with FIG. 4, along withthe spectrum 40 of the digital radio and/or television signal, a secondspectrum 45 of a second radio and/or television signal is superimposedon the otherwise unchanged spectrum 35 of the analog television signal.The two spectra 40 and 45 may for instance belong to the transmitteddigital radio signal or the transmitted digital television signal ofFIG. 1. The frequency ranges of the two spectra 40 and 45 are introducedvia the first amplifier 13 and the mixer 15 into the channel of theanalog television signal in such a way that they are separated from oneanother by a protective frequency spacing f_(S) and therefore do notimpair one another, and are superimposed on the spectrum of the analogtelevision signal between the color auxiliary carrier frequency f_(FHT)and the video carrier frequency f₂ of the neighboring channel. The firstspectrum 40 has the two audio carriers superimposed on it andinterfering with it while conversely the second spectrum 45 is locatedbetween the second audio carrier and the video carrier frequency f₂ ofthe neighboring channel.

In an analog television signal in accordance with the PAL B standard,the width of the channel is 7 MHz. This is also equivalent to thespacing between two video carrier frequencies f₁ and f₂ of adjacentchannels. For a spacing of 4.43 MHz between the video carrier frequencyf₁ and the color auxiliary carrier frequency f_(FHT,) 2.57 MHz thusremain available for occupation by one or more spectra of digital radioand/or television signals. When the aforementioned encoding algorithmsMPEG2 or MPEG4 are used, the bandwidth for the spectrum of a digitaltelevision signal can be limited to 1.5 MHz. In this way, a digitaltelevision signal can be transmitted between the color auxiliary carrierof the frequency f_(FHT) and the video carrier, located in theneighboring channel, having the frequency f₂. Alternatively, up to sixdigital radio signals can be transmitted in a frequency band 1.5 MHzwide, with data rates normally of up to 256 kbit/s. For a channel of ananalog television signal having a width of 8 MHz in accordance with thePAL G standard and a spacing of the second audio carrier frequencyf_(T2) from the video carrier frequency f₁ of 5.75 MHz, 2.25 MHz areavailable between the second audio carrier frequency f_(T2) and thevideo carrier frequency f₂ in the neighboring channel, so that a digitaltelevision signal with a spectrum 1.5 MHz wide can be transmittedbetween these two frequencies with relatively little impairment by theanalog television signals. If the spacing between the color auxiliarycarrier frequency f_(FHT) and the video carrier frequency f₁ remainsconstant at 4.43 MHz, the spacing between the color auxiliary carrierfrequency f_(FHT) and the video carrier frequency f₂ in the neighboringchannel is 3.57 MHz, so that between the color auxiliary carrier and thevideo carrier in the neighboring channel in accordance with FIG. 4, twodigital radio and/or television signals, each with a spectrum 1.57 MHzwide and a protective frequency spacing of 0.2 MHz, for instance, can betransmitted. As an alternative, instead of a digital television signal,up to six digital radio signals, with or without a protective frequencyspacing, can be transmitted, so that in the channel of the analogtelevision signal, either two television programs or one televisionprogram and six audio programs, or twelve audio programs can beaccommodated. When encoding algorithms that lead to a further reductionin the data quantities are employed, the number of television and/oraudio programs that can be transmitted can be increased.

In FIG. 5, a cable output of the broad band cable system 30 is connectedvia a converter 50, which includes a decoder 55, to an analog input 105of a television set 100 and also to a filter circuit 60, accommodated inthe decoder 55, for video carrier frequency components. The filtercircuit 60 for video carrier frequency components is connected to ademultiplexer 80 via a filter circuit 65 for audio carrier frequencycomponents, a demodulator 70, and a deciphering unit 75. Thedemultiplexer 80 is connected to a first decoder 85 and a second decoder86. The filter circuit 65 for audio carrier frequency components, thedemodulator 70, the deciphering unit 75, the demultiplexer 80, and thetwo decoders 85 and 86 are likewise accommodated in the decoder 55. Thefirst decoder 85 is connected to a digital input 110 of the televisionset 100, and the second decoder 86 is connected to a speaker 95 via anaudio amplifier 90.

The signal received from the converter 50 via the broad band cablesystem 30 includes the analog television signal, the digital televisionsignal, and the digital radio signal, which were fed into the broad banddistribution network by the circuit in accordance with FIG. 1. In theanalog input 105 of the television set 100, the received digital signalshave the effect of noise and lead to a reduction in the signal-to-noiseratio for the analog television signal. In a realization in accordancewith predetermined level values for the analog television signal and thedigital signals, however, the resultant signal-to-noise ratio istolerable. In the filter circuit 60 for video carrier frequencycomponents, interfering video carrier frequency components, especiallyfrom neighboring video carriers, are suppressed by up to 20 dB, forinstance, so that the signal-to-noise ratio for the digital signals isincreased. A corresponding suppression of audio carrier frequencycomponents of the analog television signal serves the same purpose andis effected by the filter circuit 65 for audio carrier frequencycomponents. The two filter circuits 60 and 65 can be realized by activeand/or passive components. The filtered digital signal is demodulated inthe demodulator 70, deciphered in the deciphering unit 75, and finallysplit, in the demultiplexer 80, into the digital television signal andthe digital radio signal. The two decoders 85 and 86 expand the digitalradio or television signal. The digital television signal is thenconverted into an analog signal at the digital input of the televisionset 100 and played back in the form of video and audio. The digitalradio signal is converted in the audio amplifier 90 into an analogsignal, amplified, and delivered to the speaker 95 for audio playback.

In a further exemplary embodiment, the digital-to-analog conversionalready takes place in the decoder 55, so that no digital input 110 oraudio amplifier 90 is needed in the television set 100.

FIG. 7 shows the spectrum of an analog television signal, for instancein accordance with the PAL B standard, and the width of the channel is 7MHz. Between the color auxiliary carrier frequency f_(FHT) and thefrequency f₂ of the video carrier in the upper neighboring channel, oneor more digital radio and/or television signals, combined into amultiplex signal, are superimposed on the analog television signal withthe spectrum 35. The frequency range of this multiplex signal is splitinto three frequency blocks 245, 250 and 255 that are separate from oneanother. This is accomplished by providing that in the modulation in themodulator 12, for instance by an OFDM modulation process, carrierfrequencies are selected that do not fall below a predeterminedfrequency spacing for the audio carrier frequencies f_(T1), f_(T2), thecolor auxiliary carrier frequency f_(FHT), and the video carrierfrequency f₁ of the analog television signal as well as the videocarrier frequency f₂ of the analog television signal of the upperneighboring channel. In this way, a first portion 245 of the frequencyrange of the multiplex signal is transmitted between the video carrierfrequency fFHT and the first audio carrier frequency f_(T1). It can beseen from FIG. 8 that the first portion 245 of the frequency range isseparated from the color auxiliary carrier frequency f_(FHT) by a firstprotective frequency spacing f_(S1) and from the first audio carrierfrequency f_(T1) by a second protective frequency spacing f_(S2). Asecond portion 250 of the frequency range of the multiplex signal istransmitted between the first and second audio carrier frequenciesf_(T1) and f_(T2) of the analog television signal. The second portion250 of the frequency range is separated from the first audio carrierfrequency f_(T1) by a third protective frequency spacing f_(S3) and fromthe second audio carrier frequency f_(T2) by a fourth protectivefrequency spacing f_(S4). A third portion 255 of the frequency range ofthe multiplex signal is transmitted between the second audio carrierfrequency f_(T2) of the analog television signal and the video carrierfrequency f₂ of the upper channel, that is, the channel neighboring ittoward higher frequencies. The third portion 255 of the frequency rangeis separated from the second audio carrier frequency f_(T2) by a fifthprotective frequency spacing f_(S5) and from the video carrier frequencyf₂ of the upper neighboring channel by a sixth protective frequencyspacing f_(S6). In this way, the aforementioned video and audio carrierfrequencies f_(FHT), f_(T1), f_(T2) and f₂ are prevented from beingsuperimposed on the frequency range of the multiplex signal, so thatresultant interference in reception of the digital multiplex signal isavoided.

It is also contemplated according to the invention that the describedfrequency range of the multiplex signal be used both in the forward andthe reverse directions, with the goal of using existing broad band cablesystems for interactive services as well as remote polling and/or remotecontrol tasks. The described capability of transmitting digitalsupplementary data along with a digital radio or television program canthen also be used in setting up interactive and/or telecommunicationsservices.

Finally, it is also possible to utilize the splitting up of thefrequency range into three frequency blocks 245, 250 and 255 inaccordance with FIG. 8 in such a way that one or two frequency blocksare used for transmitting digital data in the forward direction, and theremaining frequency blocks are used to transmit digital data in thereverse direction.

In FIG. 6, an apparatus 201 is shown for transmitting and receivingdigital multiplex signals modulated by an OFDM process. The multiplexsignals are preferably radio and/or television signals, but may be anypossible other digital signals, in particular those suitable forrealizing interactive and/or telecommunications services. Here bothfrequency and multiplexing can be employed. In the apparatus shown inFIG. 6, a multiplexing process is employed. In FIG. 6, 240 designates acoupler for coupling the apparatus 201 to the broad band cable system30. Connected to the coupler 240 are a tuner circuit 205 and a modulator235. Connected to the tuner circuit 205 in turn is a demodulator 210,which is connected to the decoder 215. A first channel adaptation unit230 is connected to the demodulator 235. The decoder 215 is connectedvia a demultiplexer 260 to an interface device 225, which is alsoconnected to the channel adaptation unit 230. The aforementionedcomponents are all contained within the apparatus 201.

Digital multiplex signals arriving at the apparatus 201 via the broadband cable system 30 are carried on to the tuner circuit 205 by thecoupler 240, which carries out a directional separation between signalsreceived and signals to be transmitted. In the tuner circuit 205, achannel of the transmitted frequency band is selected, and the frequencyrange of the digital multiplex signal to be received is filtered out. Ifan analog television signal is also superimposed on this frequency rangeas described above, then it is also possible for a suppression ofcorresponding video and/or audio carrier frequency components of theanalog television signal to be done in the tuner circuit 205, so as toavert resultant interference upon reception. The tuned and filtereddigital multiplex signal is also OFDM-modulated and is subjected to acorresponding demodulation in the demodulator 210. After thedemodulation, a digital, error-protected, encoded base band data streamis available, which in the decoder 215 is subjected to error evaluationand decoding, which eliminates the data redundance that is jointlytransmitted, particularly for error protection purposes, and the resultis that a decoded digital net data stream is present at the output ofthe decoder 215. The net data stream at the output of the decoder 215 issplit in the demultiplexer 260 into individual digital signals, which inturn are passed on to the interface device 225. Three double-headedarrows at the interface device 225 in FIG. 6 indicate that data playbackequipment, such as radio receivers, television sets, video cassetterecorders, telephones, personal computers, and so forth, can beconnected to the apparatus 201 via the interface device 225. An opticalbroad band distribution network can also be connected to the interfacedevice 225, preferably using fiber optics, and after electroopticalconversion in the interface device 225, this network passes on thesignals received from the apparatus 201. In the reception situation, thedata playback equipment serve the purpose of optical or acousticalplayback of the digital signals transmitted via the broad band cablesystem 30 and received by the apparatus 201. To realize interactiveand/or telecommunications services, the apparatus 201 must also beprepared for transmitting digital signals, which are transmitted to theapparatus 201 by data playback equipment via the interface device 225,on to the broad band cable system 30. The same is true for digital datawhich are carried on to the apparatus 201 via the interface device 225from an optical broad band distribution network connected to theinterface device 225. To that end, the corresponding optoelectricalconversion of the digital signals from the optical broad banddistribution network must be done in the interface device 225. Fortransmitting digital data from to the broad band cable system 30, thechannel adaptation unit 230 is provided in the apparatus 201; this unitis used for encoding and combining the individual digital signals,transmitted to the apparatus 201 via the interface device 225, into amultiplex signal. The digital multiplex signal formed in the channeladaptation unit is ofdm-modulated in the modulator 235. In this process,carrier frequencies are used that are located in frequency channelsintended for transmission of analog television signals. At the placeswhere the analog television signal to be superimposed has video andaudio carrier frequencies that interfere with reception, no carrierfrequencies are provided in the modulator 235, and thus theaforementioned six protective frequency spacings f_(S1), f_(S2 f) _(S3),f_(S4), f_(S5) and f_(S6) of FIG. 8 are observed. The omission of thecorresponding carriers is also known as frequency blanking. Because ofthis frequency blanking, the data rate of the digital multiplex signalis reduced. Via the coupler 240, the thus-modulated digital OFDMmultiplex signal is supplied to the broad band cable system 30.

What is claimed is:
 1. A method for common transmission ofdigital-modulated signals and analog-modulated signals, said methodcomprising the steps of: a) transmitting at least one digital signaltogether with an analog television signal in a common channel of atleast one communications channel; b) controlling a signal level of theat least one digital signal so that said signal level of the at leastone digital signal does not exceed a predetermined digital signal level;c) controlling a signal level of the analog television signal so thatsaid signal level of the analog television signal does not drop below apredetermined analog signal level which is much greater than thepredetermined digital signal level; d) controlling the at least onedigital signal so that an amplitude of a frequency spectrum of the atleast one digital signal does not exceed a predetermined amplitudevalue, which is much lower than an amplitude of a video carrier of theanalog television signal; and e) controlling the at least one digitalsignal so that the frequency spectrum of the at least one digital signalis limited to a frequency range smaller than a width of the commonchannel; wherein the at least one digital signal comprises at least oneof at least one radio signal and at least one television signal.
 2. Themethod as defined in claim 1, further comprising common transmission ofsaid digital-modulated and said analog-modulated signals over broadbandcable system.
 3. The method as defined in claim 1, further comprisingreducing a data quantity of the at least one digital signal by coding.4. The method as defined in claim 1, further comprising transmittingdigital supplementary data in the frequency range of the at least onedigital signal.
 5. The method as defined in claim 1, wherein thefrequency range of the at least one digital signal extends between acolor auxiliary carrier frequency of the analog television signal and avideo carrier frequency of a neighboring channel adjacent to the commonchannel and the frequency range is separated from both the colorauxiliary carrier frequency and the video carrier frequency of theneighboring by respective protective frequency spacing.
 6. The method asdefined in claim 1, wherein the analog television signal has audiocarrier frequency components and further comprising transmitting saidaudio carrier frequency components with a predetermined attenuation. 7.The method as defined in claim 1, further comprising attenuatinginterfering video and audio carrier frequency components of the analogtelevision signal by means of filter circuitry provided in a decoder fordecoding said at least one digital signal.
 8. The method as defined inclaim 1, further comprising combining a plurality of individual digitalsignals into respective blocks and transmitting said individual digitalsignals in said blocks in the at least one channel and at least two ofsaid individual signals have respective frequency ranges separated fromeach other by a protective frequency difference.
 9. The method asdefined in claim 1, wherein said at least one digital signal istransmitted in enciphered form.
 10. The method as defined in claim 1,further comprising modulating said at least one digital signal by acoded orthogonal frequency division multiplexing method to form at leastone multiplexed modulated signal and transmitting the multiplexedmodulated signal.
 11. The method as defined in claim 10, furthercomprising selecting carrier frequencies for the modulating such thatare not less than a predetermined frequency difference below an audiocarrier frequency of the analog television signal.
 12. The method asdefined in claim 10, further comprising selecting carrier frequenciesfor the modulating such that are not less than a predetermined frequencydifference below a color auxiliary carrier frequency of the analogtelevision signal.
 13. The method as defined in claim 10, furthercomprising selecting carrier frequencies for the modulating such thatare not less than a predetermined frequency difference below a videocarrier frequency of the analog television signal or a video carrierfrequency of another analog television signal in an adjacent channel.14. A method for common transmission of digital-modulated signals andanalog-modulated signals, said method comprising the steps of: a)transmitting at least one digital signal together with an analogtelevision signal in a common channel of at least one communicationschannel; b) controlling a signal level of the at least one digitalsignal so that said signal level of the at least one digital signal doesnot exceed a predetermined digital signal level; c) controlling a signallevel of the analog television signal so that said signal level of theanalog television signal does not drop below a predetermined analogsignal level which is much greater than the predetermined digital signallevel; d) controlling the at least one digital signal so that anamplitude of a frequency spectrum of the at least one digital signaldoes not exceed a predetermined amplitude value, which is much lowerthan an amplitude of a video carrier of the analog television signal;and e) controlling the at least one digital signal so that the frequencyspectrum of the at least one digital signal is limited to a frequencyrange smaller than a width of the common channel; and f) combining theat least one digital signal together with said analog television signalin a multiplex signal prior to the transmitting of step a); wherein theat least one digital signal comprises at least one of at least one radiosignal and at least one television signal and at least a portion of afrequency range of the at least one digital signal combined in themultiplex signal is transmitted between a video carrier frequency and anaudio carrier frequency of the analog television signal and is separatedfrom both the video carrier frequency and the audio carrier frequency byrespective protective frequency spacing.
 15. A method for commontransmission of digital-modulated signals and analog-modulated signals,said method comprising the steps of: a) transmitting at least onedigital signal together with an analog television signal in a commonchannel of at least one communications channel; b) controlling a signallevel of the at least one digital signal so that said signal level ofthe at least one digital signal does not exceed a predetermined digitalsignal level; c) controlling a signal level of the analog televisionsignal so that said signal level of the analog television signal doesnot drop below a predetermined analog signal level which is much greaterthan the predetermined digital signal level; d) controlling the at leastone digital signal so that an amplitude of a frequency spectrum of theat least one digital signal does not exceed a predetermined amplitudevalue, which is much lower than an amplitude of a video carrier of theanalog television signal; and e) controlling the at least one digitalsignal so that the frequency spectrum of the at least one digital signalis limited to a frequency range smaller than a width of the commonchannel; and f) combining the at least one digital signal together withsaid analog television signal in a multiplex signal prior to thetransmitting of step a); wherein the at least one digital signalcomprises at least one of at least one radio signal and at least onetelevision signal and at least a portion of a frequency range of the atleast one digital signal combined in the multiplex signal is transmittedbetween one audio carrier frequency and another audio carrier frequencyof the analog television signal and is separated from both the audiocarrier frequencies by respective protective frequency spacing.
 16. Amethod for common transmission of digital-modulated signals andanalog-modulated signals, said method comprising the steps of: a)transmitting at least one digital signal at least partially togetherwith an analog television signal in a common channel of at least onecommunications channel; b) controlling a signal level of the at leastone digital signal so that said signal level of the at least one digitalsignal does not exceed a predetermined digital signal level; c)controlling a signal level of the analog television signal so that saidsignal level of the analog television signal does not drop below apredetermined analog signal level which is much greater than thepredetermined digital signal level; d) controlling the at least onedigital signal so that an amplitude of a frequency spectrum of the atleast one digital signal does not exceed a predetermined amplitudevalue, which is much lower than an amplitude of a video carrier of theanalog television signal; e) controlling the at least one digital signalso that the frequency spectrum of the at least one digital signal islimited to a frequency range smaller than a width of the common channel;and f) combining the at least one digital signal together with saidanalog television signal in a multiplex signal prior to the transmittingof step a); wherein the at least one digital signal comprises at leastone of at least one radio signal and at least one television signal andat least a portion of a frequency range of the at least one digitalsignal combined in the multiplex signal is transmitted between a videocarrier frequency of a channel adjacent to the common channel and anaudio carrier frequency of the analog television signal and is separatedfrom both the video carrier frequency and the audio carrier frequency byrespective protective frequency spacing.
 17. The method as defined inclaim 1, further comprising combining the at least one digital signalinto a multiplex signal in any arbitrary direction.
 18. The method asdefined in claim wherein the digital-modulated signals are transmittedby a first transmitter and the analog-modulated signals are transmittedby a second transmitter.
 19. The method as defined in claim 1, whereinthe digital-modulated signals and the analog-modulated signals aretransmitted by a single transmitter.
 20. An apparatus (201) fortransmitting and receiving digital multiplex signals modulated by acoded orthogonal frequency division multiplexing method, said apparatuscomprising a receiving portion and a transmitting portion; wherein saidreceiving portion comprises a tuner circuit (205) for selecting achannel of a transmitted frequency band, a demodulator (210) fordemodulating modulated signals from the tuner circuit in said channel toform demodulated signals including demodulated digital signals, adecoder (215) for error evaluation of the demodulated digital signals, ademultiplexer (260) for splitting the demodulated signals intoindividual digital signals and an interface device (225) fortransferring the individual digital signals to data playback devices andan optical broadband distribution network; and wherein said transmittingportion comprises a channel adaptation unit (30) for combining andcoding individual digital signals received from at least one of the dataplayback devices and the optical broadband distribution network via saidinterface device (225) to form multiplex signals; a modulator (235) formodulating the multiplex signals from the channel adaptation unit atcarrier frequencies in predetermined frequency channels and a coupler(240) for connection to a broadband cable system (30), said coupler(240) being connected to the tuner circuit (205) and the modulator(235).
 21. The apparatus as defined in claim 20, wherein the frequencychannels for the multiplex signal are for transmission of the analogtelevision signal.
 22. The apparatus as defined in claim 20, wherein themodulator (235) includes means for reducing a data rate of the multiplexsignal and the multiplex signal has signal components to be modulated atselected carrier frequencies that exceed by a predetermined frequencyspacing the greatest of at least one audio carrier frequency, at leastone color auxiliary carrier frequency or at least one video carrierfrequency of the analog television signal.